This invention relates to a method and apparatus for producing high purity tantalum and the high purity tantalum so produced. In particular, the invention relates to production of high purity tantalum.
Tantalum is currently used extensively in the electronics industry which employs tantalum in the manufacture of highly effective electronic capacitors. This is mainly attributed to the strong and stable dielectric properties of the oxide film on the anodized metal. Both wrought thin foils and powders are used to manufacture bulk capacitors. In addition, thin film capacitors for microcircuit applications are formed by anodization of tantalum films, which are normally produced by sputtering. Tantalum is also sputtered in an Arxe2x80x94N2 ambient to form an ultra thin TaN layer which is used as a diffusion barrier between a Cu layer. and a silicon substrate in new generation chips to ensure that the cross section of the interconnects can make use of the high conductivity properties of Cu. It is reported that the microstructure and stoichiometry of the TaN film are, unlike TiN, relatively insensitive to the deposition conditions. Therefore, TaN is considered a much better diffusion barrier than TiN for chip manufacture using copper as metallization material. For these thin film applications in the microelectronics industry, high purity tantalum sputtering targets are needed.
Most of the tantalum metal produced in the world today is derived from sodium reduction of potassium heptafluotantalate (K2TaF7). Processes which are not adapted commercially to any significant extent include the reduction of tantalum oxide (Ta2O5)with metallic reductants such as calcium and aluminum, and non metallic reductants carbon and carbon nitrogen; the reduction of the tantalum pentachloride (TaCl5) with magnesium, sodium or hydrogen; and the thermal dissociation of TaCl5.
Reduced tantalum is obtained either as powder, sponge or massive metal. It invariably contains significant amounts of oxygen, as well as other impurities such as reductants and impurities that may be present in the starting tantalum compounds. For removal of impurities in tantalum, electron beam melting is often conducted. During electron beam melting, most of the metallic impurities and interstitial gases are vaporized because of their high vapor pressure at the melting point of tantalum (2996xc2x0 C.). Essentially all elements, except niobium, tungsten, molybdenum, uranium and thorium can be eliminated this way. While the metallic impurities and nitrogen are removed by direct volatilization, the removal of oxygen takes place via mechanisms involving formation and evaporation of carbon oxides, aluminum oxides, water, as well as suboxides of tantalum. The purity can be further improved by repeated electron beam melting. Other refining processes include vacuum arc melting, vacuum sintering, molten salt electrorefining and tantalum iodide refining, with the iodide process being the most promising technique for removing tungsten and molybdenum.
The above mentioned refining methods are not effective for removal of niobium from tantalum. Since tantalum and niobium are closely associated with each other in nature, the removal of niobium is critical to prepare very high pure tantalum. In practice, their separation is conducted before reduction via methods such as solvent extraction, chlorination and fractional crystallization.
The tantalum target manufacturing process includes forging ingot into billet, surface machining billet, cutting billet into pieces, cold rolling the pieces into blanks, annealing blanks. final finishing and bonding to backing plates.
In accordance with the present invention there is provided a method and apparatus for producing high purity tantalum sputtering targets and the high purity tantalum so produced.
The method comprises purifying potassium heptafloutantalate, K2TaF7, reducing the purified K2TaF7 to produce tantalum powder, refining the tantalum by reacting with iodine and finally electron beam melting the tantalum to form a high purity tantalum ingot.
The starting material is commercial K2TaF7 salt, made by dissolving tantalum ores in hydrofluoric and sulfuric acid mixture, followed by filtration, solvent extraction using methkylisobutylketone (MIBK) and crystallization of K2TaF7. This can be repeated several times to lower the impurity levels, in particular the level of Nb.
Sodium reduction of purified K2TaF7 is conducted in a liquid liquid reduction retort where K2TaF7 and diluents (KCl and NaCl) are heated to about 1000xc2x0 C. Molten sodium is then injected into the retort for reacting with K2TaF7. Agitation of the reactants is provided to accelerate the reduction reaction. After cooling, the mass is taken out of the retort. crushed. leached and washed to separate tantalum powder from the salt mixture.
Tantalum refining is done by the iodide process or electron beam melting. These methods can be used in parallel or in series. Electron beam melting is preferred as the last step because it results in an ingot which is suitable for further physical metallurgical steps toward the goal of target manufacture.
Electron beam melted ingot is forged into billets and surface machined. After surface machining, the forged billet is cut into pieces, which are further cold rolled into blanks. Blank annealing is carried out in an inert atmosphere to obtain a recrystallized microstructure. The blanks are then machined to obtain a final finish and bonded to copper or aluminum backing plates.
For characterization of targets produced by the invented process, chemical analyses are conducted. The methods of chemical analysis used to derive the chemical descriptions set forth herein are the methods known as glow discharge mass spectroscopy (GDMS) for metallic elements and LECO gas analyzer for non metallic elements.
The highly purified tantalum material of the present invention has less than 500 ppm by weight, total metallic impurities, an oxygen content of less than about 100 ppm, by weight, a molybdenum or tungsten content of not more than 50 ppm, by weight, and a uranium and thorium content of not more than 10 ppb, by weight. It is also possible to produce tantalum having less than 5 ppm, by weight, total of molybdenum and tungsten.